1. Field of the Invention
This invention relates to plant nutrient supplements and animal feed supplements and to a process for preparing same. More particularly, the invention relates to such products and an improved process for preparing a substantially free-flowing and non-caking particulate urea-calcium sulphate nutrient supplement which is readily blended with one or more nutrients to provide fertilizers or animal feeds.
2. Description of the Prior Art
Plant nutrients can be classified as primary plant nutrients (sources of nitrogen, phosphorous and potassium the last mentioned being measured as K.sub.2 O); secondary plant nutrients (sources of calcium, magnesium and sulfur); and micronutrients (such as copper, zinc, boron, iron, manganese, molybdenum and the like). Fertilizer nitrogen (hereinafter N) is normally provided by ammonia, ammonium salts such as ammonium nitrate, or urea. Fertilizer phosphate (hereinafter P and measured as P.sub.2 O.sub.5) is normally provided by phosphoric acid, ammonium phosphates, and super phosphates.
For certain fertilizer and animal feed formulations, it is often desirable to incorporate urea along with a phosphate compound such as dicalcium phosphate or monoammonium phosphate and to include calcium and sulfur. Dicalcium phosphate is the most common source of phosphate used in animal feed preparations, and monoammonium phosphate is widely used in fertilizer formulations. Gypsum (calcium sulphate dihydrate or land plaster) is frequently used in both formulations as a calcium source, a sulfur source or both.
In recent years in the animal feeding area, it is becoming accepted to substitute urea (a non-protein nitrogen) for a portion of preformed protein in formulating diets for ruminants, particularly cattle since they are able to synthesize essential sulfur containing amino acids from inorganic sulfur and urea. There has further been a tendency to use higher levels of urea and an increasing interest in using gypsum in feeding these animals.
However, as is well known in the art, urea is very water soluble and is quite hygroscopic; and urea-gypsum and urea-phosphate mixtures have a history of handling problems. Urea, whether marketed in the form of crystals or prills has a great tendency to cake and form a rigid mass when stored for any significant period of time. Thus, the fertilizer manufacturer, the feed lot operator and the ultimate consumer must crush or otherwise break up this mass before the urea can be combined with other animal feed ingredients or fertilizer ingredients or before the blended products can be used for their intended purposes. Further urea prills are of such large size that they must be intimately ground for thorough blending with animal feed ingredients to avoid "wetting out" in feed pellets and other problems. When the urea is co-ground, or simply mixed, with hydrated compounds having loosely bound water of crystallization such as dicalcium phosphate, monocalcium phosphate, or gypsum for blending or granulation processing, the urea dehydrates these materials. That is, the urea reacts with and liberates the bound water from these compounds, thereby wetting out the mixtures, even when the compounds are present in dried powder or particulate form, and consequently results in the formation of new adduct compounds. These mixtures then have poor storage properties, i.e., they become wet and soft, and caking results from the adduct interaction. Thus additional time and labor is required in order to render the material suitable for even distribution in formulation or application. Additionally, the materials tend to segregate during handling and storage, thus requiring additional time and labor in remixing to form a homogeneous mixture.
Various methods have been proposed for avoiding some of the above noted caking and related handling problems with urea. One prior proposal involves "dusting" a conditioning agent onto the urea (such as starch, clays, calcium sources as recited in the background of U.S. Pat. No. 3,332,827) or the in situ coating of completed fertilizers containing urea with calcium phosphates or calcium sulphates (U.S. Pat. No. 3,419,379). Other proposals involve the intentional formation of the various addition complexes or ostensible alternating coatings of addition complex ingredients together with high temperature processing in order to form hard outer shells to the granules (U.S. Pat. Nos. 2,074,880; 3,085,870 and 3,585,043 and also 1,977,628 prior to prilling of the urea). Although many such proposals have been made, none has proved entirely satisfactory. Thus, granules formed by mixing the materials and then prilling the urea have been weak and readily disintegrating to powders in handling or processing them. The use of addition complexes not only requires added production equipment, but the hard granules produced from such mixtures have poor storage properties and frequently upset the delicate balance of blending nutrient sources such that a complete animal feed or fertilizer of properly balanced source materials cannot result. Usually the complex salts formed from urea admixtures in fertilizer and animal feed products are more hygroscopic than urea alone. The concept of alternative coatings suffers from the detriment of still being hygroscopic, and caking results. Gypsum-urea addition complexes that are less hygroscopic are likely to contain too much sulfur source to be utilized for animal feed supplements or secondary plant nutrient addition; and when the ratios are adjusted to provide optimum N:S ratios, the resultant process mixtures produced by pelletizing and granulating cakes almost as severely as urea alone. For example a 10N:1S ratio prepared from a urea:gypsum mixture has been found to cake just as badly as does the urea alone. Other addition complexes that are less hygroscopic than urea alone may have lower melting points, such that additional processing can cause a complete gumming up and stoppage of the processing line.